Field of the Invention
The present invention relates to a hydraulic control system for a construction machine.
Description of the Related Art
A hydraulic control system for a construction machine such as a hydraulic excavator generally comprises a hydraulic pump, hydraulic actuators driven by hydraulic fluid delivered from the hydraulic pump, and control valves for controlling the supply and discharge of the hydraulic fluid to/from the hydraulic actuators. For example, the hydraulic actuators of a hydraulic excavator include a boom cylinder, an arm cylinder and a bucket cylinder for respectively driving a boom, an arm and a bucket of a front work implement. Each hydraulic actuator is provided with a corresponding control valve.
Each of these control valves has a meter-in restrictor and a meter-out restrictor. The flow rate of the hydraulic fluid supplied from the hydraulic pump to the hydraulic actuator is controlled by the meter-in restrictor, while the flow rate of the hydraulic fluid discharged from the hydraulic actuator to a tank is controlled by the meter-out restrictor.
Incidentally, attachments that can be attached to the front work implement include not only a standard bucket but also a large-sized bucket, a crusher, a pulverizer, etc. Many of these attachments are heavier than the standard bucket. Thus, if an opening area of the meter-out restrictor is adjusted in the development phase assuming the state in which the standard bucket is attached to the front work implement, the following problems arise when the user attaches a heavyweight attachment to the front work implement instead of the standard bucket:
When the arm equipped with an attachment is pulled in the air, that is, crowded, a load is placed on the arm cylinder due to the weight of the arm and the attachment as a load in the same direction as the expanding direction of the arm cylinder (hereinafter referred to as a negative load). When a heavyweight attachment is attached to the front work implement, this negative load increases in comparison with the state in which the standard bucket is attached. Therefore, it is presumed that the operating speed of the arm cylinder increases in comparison with the state in which the standard bucket is attached and a breathing phenomenon (cavitation) occurs due to an insufficient flow rate of the hydraulic fluid on the meter-in side. As a result, the operability of the construction machine deteriorates. Incidentally, the breathing phenomenon can cause damage to the hydraulic actuators and the control valves.
To prevent the occurrence of such problems, the opening area of the meter-out restrictor is usually adjusted assuming the state in which a heavyweight attachment is attached. However, when work is performed by attaching the standard bucket to such a construction machine whose meter-out restrictor opening area has been adjusted as above, meter-out pressure loss greater than that necessary for supporting the negative load due to the weight of the arm and the standard bucket occurs on the meter-out side and that leads to energy loss of the construction machine.
To resolve this problem, there exists a hydraulic circuit that is provided with pressure-reducing means for reducing pilot pressure acting on a pilot pressure receiving part on the arm cylinder expansion side of the arm flow control valve depending on the pressure of the hydraulic fluid supplied to the arm cylinder and is configured to regulate the spool stroke of the arm flow control valve and thereby reduce the opening area of the meter-out restrictor when the arm crowding operation is performed in the air (see JP-2012-36665-A, for example).